1. Field of the Invention
This invention relates to a high voltage generating cirucit for an automobile ignition system, and more particularly to such a high voltage generating circuit for applying high frequency pulse signals to plural spark plugs during each spark plug ignition.
2. Description of the Prior Art
High voltage generating circuits of the above-noted type are known in the prior art, as for example disclosed in U.S. Pat. No. 4,245,594 to Morino et al. While the prior art high voltage generating circuits perform well under some engine operating conditions, nevertheless an ignition current within an optimum range of about 30-100 mA cannot always reliably be obtained at the spark plug because the spark sustaining voltage changes in accordance with various engine conditions while the supply voltage is constant. In the event that an ignition current less than 30 mA is realized, than one cannot obtain adequate engine performance and engine emissions increase. Ignition currents higher than 100 mA on the other hand results in early spark plug wear out and tend to destroy circuit elements within the ignition circuit.
FIG. 1 illustrates the spark plug ignition current I.sub.s versus sustaining voltage V.sub.s characteristic. The curves "a", "b" and "c" indicate I.sub.s -V.sub.s relations under different engine conditions. Curve "b", for example, indicates a very stable voltage V.sub.s within a very wide current range I.sub.s when a current is supplied to the spark plug from a fixed power supply source. Circuit parameters are then selected for operation at a selected I.sub.s -V.sub.s point, for example, at point A shown in FIG. 1, considering the voltage level of the fixed power supply source. However, if engine conditions, such as vacuum pressure, temperature etc., change, the curve "b" may be shifted to the right to curve "c", where I.sub.s becomes zero, or on the other hand may be shifted to curve "a" where I.sub.s becomes very large. However, as above noted, when I.sub.s is below the optimum range, the efficiency of fuel consumption decreases and the amount of pollutants in the exhaust gases is increased, and when I.sub.s is above the optimum range, early spark plug wear out occurs and ignition circuit elements experience a greater number of failures.